rrdtool update does not update one of my ds

In my graph I have the following datasources:

DEF:a="/usr/share/cacti/rra/sx2db00_mt_gnuf_com_avgqu_sz_7020.rrd":'avgqu_sz':AVERAGE \
DEF:b="/usr/share/cacti/rra/sx2db00_mt_gnuf_com_avgqu_sz_7020.rrd":'svctm':AVERAGE \

Looking at the graph, only one of the data sources are graphed, and
when I put in a GPRINT I see that one is NaN. Checking the log for
that host and searching for updates, I see this:

04/03/2013 09:00:04 AM - POLLER: Poller[0] CACTI2RRD: /usr/bin/rrdtool update /usr/share/cacti/rra/sx2db00_mt_gnuf_com_avgqu_sz_7020.rrd --template svctm N:1.11
04/03/2013 09:05:05 AM - POLLER: Poller[0] CACTI2RRD: /usr/bin/rrdtool update /usr/share/cacti/rra/sx2db00_mt_gnuf_com_avgqu_sz_7020.rrd --template svctm 1364979902:13.60
04/03/2013 09:10:04 AM - POLLER: Poller[0] CACTI2RRD: /usr/bin/rrdtool update /usr/share/cacti/rra/sx2db00_mt_gnuf_com_avgqu_sz_7020.rrd --template svctm 1364980201:2.35
04/03/2013 09:15:05 AM - POLLER: Poller[0] CACTI2RRD: /usr/bin/rrdtool update /usr/share/cacti/rra/sx2db00_mt_gnuf_com_avgqu_sz_7020.rrd --template svctm 1364980501:2.42
04/03/2013 09:20:04 AM - POLLER: Poller[0] CACTI2RRD: /usr/bin/rrdtool update /usr/share/cacti/rra/sx2db00_mt_gnuf_com_avgqu_sz_7020.rrd --template svctm 1364980801:3.23

So it looks like one ds just isn't updated. Running the Data Input method script I see that I get data like this:
avgqu_sz:0.40 svctm:3.54

I have confirmed by looking in the log that the script is run by the poller and that kind of data gets output.

And my data template has both of those two ds' defined. 

Any hints on what went wrong?

-andreas

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